1. Cross Reference to Related Application
The present application is related to applicant's copending application Ser. No. 742,846 filed Aug. 8, 1991.
2. Field of the Invention
This invention relates to titanium-aluminum-based (TiAl-based) intermetallic compound alloys and processes for preparing the same. More particularly, this invention relates to TiAl-based intermetallic compound multi-component systems with high superplastic deformability and strength, containing chromium as a third major element. The TiAl-based intermetallic compound alloys according to this invention are used for heat-resistant structural materials requiring high specific strength.
3. Description of the Prior Art
Though much expectation is entertained as a heat-resisting material, TiAl intermetallic compound alloys are difficult to work due to low ductility. This low workability, a chief obstacle to the use of TiAl, can be improved by two methods; i.e. application of appropriate working method and preparation with proper alloy component design. The low workability is generally due to the lack of ductility at room temperature. Even at higher temperatures, however, the workability of TiAl alloys remains unimproved and, therefore, rolling, forging and other conventional working processes cannot be applied directly.
Applicable working processes include near-net-shaping, a typical example of which being powder metallurgy, and modified forms of rolling, forging and other conventional working processes including sheath and isothermal rolling. Forming by high-temperature sheath rolling (at a temperature of 1373K and a speed of 1.5 m/min.) of Co-based superalloy (S-816) (Japanese Provisional Patent Publication No. 213361 of 1986) and shaping by isothermal forging at a temperature of 800.degree. C. (1073K) or above and a strain rate of 10.sup.-2 sec.sup.-1 or under (Japanese Provisional Patent Publication No. 171862 of 1988) have been reported. These processes achieve forming and shaping by taking advantage of a characteristic property of TiAl to exhibit ductility at 800.degree. C. (1073K) together with the strain-rate sensitivity of the mechanical properties of TiAl. Still, they are unsuitable for mass production because the temperature must be kept above 1273K and the strain rate must be kept as low as possible for the achievement of satisfactory forming and shaping. Another shaping process reported subjects a mixed compact of titanium and aluminum to a high temperature and pressure (Japanese Provisional Patent Publication No. 140049 of 1988). While this process has an advantage over those mentioned before that not only primary shaping but also various secondary shaping can be accomplished, the use of active titanium and aluminum unavoidably entails mixing of unwanted impurities.
Several processes to improve the ductility at room temperature by the addition of elements have been also reported. While the National Research Institute for Metals of Japan proposed the addition of manganese (Japanese Provisional Patent Publication No. 41740 of 1986) and silver (Japanese Provisional Patent Publication No. 123847 of 1983), General Electric Corporation proposed the addition of silicon (U.S. Pat. No. 4,836,983), tantalum (U.S. Pat. No. 4,842,817), chromium (U.S. Pat. No. 4,842,819) and boron (U.S. Pat. No. 4,842,820). The contents of silicon, tantalum, chromium and boron in the alloy systems proposed by General Electric Corporation are determined based on the bending deflection evaluated by the four-point bend test. The content of titanium in all of them is either equal to or higher than that of aluminum. Other examples of improved ductility at high temperatures reported include the addition of 0.005% to 0.2% by weight of boron (Japanese Provisional Patent Publication No. 125634 of 1988) and the combined addition of 0.02% to 0.3% by weight of boron and 0.2% to 5.0% by weight of silicon (Japanese Provisional Patent Publication No. 125634 of 1988). For the improvement of other properties, addition of more elements must be considered. Addition of elements to improve not only ductility but also, for example, oxidation and creep resistance necessitates extensive component adjustment. A tensile elongation of 3.0% at room temperature is considered as a measure of adequate ductility. But this level has not been achieved by any of the conventionally proposed alloys. To achieve that high level of ductility, as such, grain refinement and other microstructure control measures must be taken together with the application of properly selected working processes.